In-situ LA-SF-ICP-MS U-Pb dating and geochemistry of garnets from Hongniu-Hongshan skarn-type copper deposit in northwestern Yunnan

Hongniu-Hongshan copper deposit is the largest(distal)skarn type deposit discovered in the south section of Yidun Arc.However, there is no direct chronological evidence for the diagenetic age of ore-bearing skarn.The accurate determination of the diagenetic age of skarn is of great significance to the location of this deposit in the two stages of Mesozoic metallogenic events of porphyry in the south section of Yidun Arc.The garnets in skarn of the deposit can be divided into two stages, where the early garnets(Grt Ⅰ)are mostly found in skarn of lean ore with coarse grain structure and isotopic body; and the late garnets(GrtⅡ)are mostly found in skarn of rich ore with fine-medium grain structure, oscillatory zoning and heterogeneous and optical anomaly.Based on in-situ LA-SF-ICP-MS U-Pb dating and electron microprobe in-situ concentrations analysis of two stages of garnets in this paper, the U-Pb ages of GrtⅠ and GrtⅡ are 84.2±3.0 Ma and 81.7±3.5 Ma, respectively.The contents of major and trace elements suggest that the garnets of the two stages are classified in the andradite and grossularite series, lack of Rb, Ba, K, Sr, etc.and rich in elements Th, U and P, where GrtⅠare characterized by relatively rich in andradite, high LREE/HREE ratio and high positive anomaly of Eu, while GrtⅡ features relatively rich in grossularite, low LREE/HREE ratio and no abnormal Eu.In combination of the Late Yanshanian mountain metallogenic events of regional collisional porphyry, it is deemed that the diagenetic and metallogenic period of distal skarn in this deposit is at about 80 Ma, when that mineralization is closely related to the acidic porphyry over the same period.The geochemical characteristics of garnets reveal that the dry skarn stage is characterized by a high water/rock ratio and dominated by infiltration metasomatism.The early fluid developed into a relatively closed system of oxidation, high temperature, rich iron and weak alkalinity, and the late stage formed an open system of a relative reduction, relatively low temperature, rich aluminum and acid.